Electroplating industry causes heavy pollution, and the composition of the wastewater therefrom is complex due to the numerous electroplating types and the troublesome process. The pollution caused by electroplating wastewater is mainly characterized by coexistence of heavy metals and organic pollutants. A variety of carcinogenic, teratogenic and mutagenic or highly toxic substances are present in the electroplating wastewater. In recent decades, the pollution control techniques against heavy metals and organic substances in the electroplating wastewater, such as chemical precipitation, adsorption, biological method, electrolysis, membrane separation and ferrite, have been widely researched and applied. However, with the implementation of the new Discharge Standard for Pollutants from Electroplating (GB21900-2008), the discharge standard cannot be met if the electroplating wastewater is treated following the existing conventional treatment methods. In view of the state of the art, a single technology is hard to meet the high discharge standard, so it is necessary to develop an economical and efficient combined process.
Bio-contact oxidation technology is a biofilm process functioning between the activated sludge process and the biological filter process, and characterized by arranging a packing in the tank, oxygenating the sewage through aeration at the bottom of the tank, and allowing the sewage in the tank to be in a flowing state, to ensure the full contact between the sewage and the packing in the sewage, and avoid the defects of insufficient contact between the sewage and the packing in the bio-contact oxidation tank. The bio-contact oxidation process has found wide use in printing and dyeing, oily, electronic and electroplating wastewater with high salinity and non-biodegradable organic substances, as described in published literatures, for example, Study on the Experiment of High Salinity Wastewater Treatment with Bio-Contact Oxidation System, Applied Chemical Industry, Vol. 37, No. 11, 2008; Pulse Water-Lifting Aeration Bio-Contact Oxidation Process for Purification of Micro-Polluted Water, China Water & Wastewater, Vol. 27, No. 19, 2011; Treatment of Electroplating Plant Wastewater by Fenton Reagent and Biological Contact Oxidation process, Guangdong Chemical Industry, Vol. 38, No. 7, 2011; and Microelectrolysis/Plate Setter/Biological Contact Oxidation Process for Treatment of Printed Circuit Board Wastewater, Recyclable Resources and Circular Economy, Vol. 4, No. 9, 2011.
Adsorption is an important advanced treatment technology for chemical tail water. However, the activated carbon is difficult to be regenerated, and is costly in industrialization. As a synthetic adsorbing material, resin has the advantage of being renewable, as compared with the activated carbon. Electroplating wastewater is characterized by high salinity and poor biodegradability. Therefore, after the bio-contact oxidation process with the carbon nutrient source added to the electroplating wastewater post pretreatment, advanced treatment is required to further improve the water quality of the effluent.
Magnetic resin is a magnetic macromolecule polymer, and can be quickly separated from water by means of the magnetism. Besides, the magnetic resin has a small diameter of generally less than 0.2 mm, such that the pollutants are easier to diffuse into the internal porous channel of the magnetic resin. In this regard, a series of research and development efforts are made by the ORICA Company, Australian and Nanjing University. The effects of the application of magnetic resins in water treatment are described in published literatures, for example, Application of MIEX Resin in Acrylic Wastewater Treatment, Industrial Safety and Environmental Protection, No. 01, 2010.
A chelating resin can selectively adsorb heavy metal ions in wastewater. Literatures show that the chelating resin has a higher selectivity than a common cation exchange resin, as described in, for example, Adsorption of Metal Cation by Chelating Resin, Technology of Water Treatment, Vol. 37, No. 1, 2011; Progress of Adsorption of Precious Metal Ions of Chelating Resins, Applied Chemical Industry, Vol. 39, No. 5, 2010.
As described in Cheng Meifen et al. Study on the Treatment of Electroplating Wastewater by Air Floatation-Biochemical Treatment-Coagulative Precipitation Process, Industrial Wastewater Treatment, Vol. 30, No. 2, 2010, the CODcr in the effluent after treatment is 80 mg/L or below. As described in Zhao Jing et al. Study on Treatment of Copper and Nickel Electroplating Wastewater by Fenton Oxidation-Biological Aerated Filter Process, Electroplating & Finishing, Vol. 29, No. 4, 2010, the CODcr in the effluent after treatment is 80 mg/L or below, the concentration of Cu2+ is 0.5 mg/L or below and the concentration of Ni2+ is 0.5 mg/L or below. As described in Wang Gang et al. Treatment of Electroplating Plant Wastewater by Fenton Reagent and Biological Contact Oxidation Process, Guangdong Chemical Industry, Vol. 38, No. 7, 2011, the quality indexes of the effluent are as follows: CODcr≦80 mg/L, Cu2+≦0.5 mg/L, Ni2+≦0.5 mg/L, Cr6+≦0.4 mg/L and cyanide≦0.4 mg/L. As described in Yao Ting, Microelectrolysis/Plate Setter/Biological Contact Oxidation Process for Treatment of Printed Circuit Board Wastewater, Recyclable Resources and Circular Economy, Vol. 4, No. 9, 2011, the COD and the concentrations of Cu2| and Ni2| in the effluent can only meet the Class I discharge standard in the Integrated Wastewater Discharge Standard (GB8978-1996). Accordingly, the existing literatures show that although the combination of the bio-contact oxidation and various processes in the prior art can achieve high removal rate of the pollutants in the electroplating wastewater, the water quality of the effluent can only meet the Class I discharge standard in the Integrated Wastewater Discharge Standard (GB8978-1996) or approximate the new Discharge Standard for Pollutants from Electroplating (GB21900-2008), that is, the major pollution indexes: CODcr≦80 to 100 mg/L, Cu2+≦0.5 mg/L, and Ni2+≦0.5 mg/L.
With the increasingly rigorous environmental protection standards, especially in regions with high territorial development density, decreased environmental carrying capacity, or low water environmental capacity and fragile ecological environment, the local wastewater discharge standards are even stricter. It is still a difficulty in research to develop an economical and efficient combined advanced treatment technique for electroplating wastewater.